Technical Field
The present disclosure relates to a frequency-variable LC filter including a resonance circuit of an inductor and a variable capacitor.
Patent Document 1 discloses a frequency-variable LC filter using an inductor and a variable capacitor. The frequency-variable LC filter in Patent Document 1 includes a first LC parallel circuit, a second LC parallel circuit, a third series circuit, and a fourth series circuit. Both of the first LC parallel circuit and the second LC parallel circuit include parallel circuits of inductors and variable capacitors. One ends of the first LC parallel circuit and the second LC parallel circuit are connected to each other with a coupling inductor and the other ends thereof are connected to a ground potential.
The third series circuit includes a variable capacitor, and one end thereof is connected to the first LC parallel circuit and the other end thereof is connected to a first connection terminal. The fourth series circuit includes a variable capacitor, and one end thereof is connected to the second LC parallel circuit and the other end thereof is connected to a second connection terminal.
The one end (end portion at the side at which the coupling inductor is connected) of the first LC parallel circuit is connected to the first connection terminal with a first variable capacitor interposed therebetween. The one end (end portion at the side at which the coupling inductor is connected) of the second LC parallel circuit is connected to the second connection terminal with a second variable capacitor interposed therebetween. The first connection terminal and the second connection terminal are connected with a fixed capacitor interposed therebetween.
With this configuration, capacitances of the variable capacitors of the first and second parallel circuits and capacitances of the variable capacitors of the third and fourth series circuits are changed to adjust bandpass characteristics.
Patent Document 1: International Publication No. 2005/088832